Paint formulations based on alkyd resins are known. An alkyd resin can be defined as the reaction product of a dibasic acid or anhydride and a polyol to which sufficient mono-functional acid or alcohol has been added to prevent gelation during processing. If an excess of dibasic acid or anhydride relative to the polyol is used, an alcohol will generally be used to esterify the acidic carboxyl groups of the reaction product of the acid or anhydride and the polyol. If, on the other hand, an excess of the polyol is used, a monofunctional acid will be used to esterify the free hydroxyl groups. Commonly applied dibasic acids and anhydrides are maleic acid and phthalic acid and their anhydrides, while commonly applied polyols are pentaerythritol and glycerol. The monofunctional acid or alcohol used for esterification purposes usually is a long chain fatty acid or alcohol. Usually a long chain fatty acid or a mixture of several long chain fatty acids is used in the form of an oil, i.e. esterified with glycerol into a triacylglycerol that melts below room temperature. Examples of commonly applied oils are linseed oil and soybean oil. The alkyd resin thus is a polymeric network of chemically linked dibasic acid or anhydride, polyol and monofunctional acid or alcohol entities. The term "long oil" in long oil alkyd resins indicates that a relatively high amount of oil has been incorporated into the alkyd resin, i.e. approximately between 55 and 80% by weight based on total weight of alkyd resin.
The alkyd resin is responsible for an increased hardness of the final paint layer after drying, thus making it water-resistant and outdoor-durable. It also contributes to the gloss of the paint layer and the color retention of the paint.
Alkyd resins as such, however, are stiff and highly viscous at room temperature. In order to get a processable paint it is therefore necessary that a solvent is added. Suitable solvents must on the one hand have sufficient solvency power to dissolve the alkyd resin and must, on the other hand, have an appropriate boiling point or boiling point range in order to be sure that the paint layer becomes dry within an acceptable period of time. It is therefore essential that the solvent is inert towards the substrate and evaporates from the paint layer within a reasonable time, or, in other words, the solvent must have such vapor pressure at ambient temperature that it can pass through and leave the polymeric network forming the paint layer sufficiently fast. It will be understood that a too short drying time is undesirable from a processing point of view, since this would make it difficult to spread the paint evenly over its substrate. A too long drying time is also undesired, since this would cause the paint to start dripping after having been brought onto its substrate. A too long drying time would also make it very time-consuming to cover a substrate with more than one paint layer and it would cause the paint layer to be soft and sticky for a relatively long period, thus leading to a paint layer of inferior quality (beside dripping, pick up of dust, less gloss and decreased hardness). Hence, the choice of the solvent is very important.
The traditional solvent used for paint formulations is white spirit, which is a mixture of alkanes, cycloalkanes and aromatic compounds and is obtained as a distillation fraction from crude oil. Its boiling point range runs from approximately 150.degree. C. to 200.degree. C. at atmospheric pressure. An example of a commercially available and widely used white spirit is LAWS (Trade Mark). White spirit contains significant amounts of aromatic hydrocarbons. In view of the rather penetrating odor of these solvents, for which the aromatic hydrocarbons are mainly responsible, as well as from environmental considerations, there have been proposed in the past other solvents containing no or only low amounts of aromatic hydrocarbons. Despite the generally recognized principle that the order of solvent power or cutting power decreases from aromatics to cycloalkanes to alkanes, cycloalkanes, isoparaffins (i.e. branched alkanes) and dearomatized white spirits have been used as solvents for specific resin systems, including long oil alkyd resin systems. Such solvents are commercially available. For instance, a widely used commercially available dearomatized white spirit is SHELLSOL D40 (SHELLSOL is a Trade Mark). In general, for alkyd resin-based paint formulations the cutting power of a solvent is the amount of alkyd resin which can be dissolved in the solvent while the solution has the viscosity to be readily processed using a brush. A solvent having a high cutting power is advantageous, since this implies that only a little amount of this solvent is needed for obtaining the right paint viscosity.
Dearomatized white spirit is white spirit from which substantially all the aromatics have been removed or of which substantially all the aromatics have been converted into cycloalkanes. Due to the presence of cycloalkanes, the dearomatized white spirit has good cutting power. However, a major disadvantage is the penetrating odor primarily caused by the cycloalkanes.
Cycloalkanes have a slightly lower cutting power than aromatic hydrocarbons, so that more cycloparaffinic solvent is needed for dissolving an amount of long oil alkyd resin than would be the case when an aromatic solvent is used. Their penetrating odor, however, is the major disadvantage as far as their use as solvents in paint formulations is concerned.
Isoparaffins are normally produced by alkylation reactions between C.sub.3 and C.sub.5 alkenes and the corresponding alkanes followed by hydrogenation. A wide range of isoparaffinic solvents is available on the market. Although isoparaffinic solvents are useful in paints, the viscosity and the retention properties of isoparaffinic solvents are not optimal in relation to their cutting power.
One known way of adjusting the viscosity of isoparaffinic solvents to the desired value is the addition of viscosity modifiers, which typically are oxygenated hydrocarbons. Another drawback of isoparaffinic solvents is the branched molecular structure, which causes them to diffuse more slowly through the alkyd resin network than linear molecules. This is particularly relevant for the last stage of the drying process when the last part of the solvent has to leave the paint layer. After most of the solvent has vanished via evaporation, a small quantity will remain trapped in the paint layer. For optimum quality, as little solvent as possible should be left in the paint layer. Usually a part of the remaining solvent will leave the paint layer via diffusion through the alkyd resin network towards the surface of the paint layer. The branched structure of isoparaffin molecules hinders that diffusion, so that a relatively large part of the trapped solvent molecules will remain in the paint layer.
It is therefore an object of the invention to provide long oil alkyd resin-based paint formulations having improved viscosity and retention properties as compared with isoparaffinic solvent-based long oil alkyd resin-based paint formulations. More specifically, the present invention aims to provide long oil alkyd resin-based paint formulations, wherein the solvent used (i) is essentially free of aromatic and cycloaliphatic hydrocarbons and other impurities, (ii) gives the paint formulation the appropriate viscosity without the need of using viscosity modifiers and (iii) has excellent evaporation properties, i.e., has a suitable boiling point range.